General philosophy of probability theory
Probability is central to science, more than any other part of math. It enters statistics, physics, biology, and even medicine as we will see when and if we discuss tomography. This is the broad view.
There is also a narrow view – one needs to understand it before one can effectively apply it and it has many subtleties. Possibly this is due to the fact that probability, stochasticity, or randomness, may not actually exist! I think it mostly exists in our uncertainty about the world. The real world seems to be deterministic (of course one can never test this hypothesis). It is chaotic and one uses probabilistic models to study it mainly because we don’t know the initial conditions. Einstein said that ”god does not play dice”. My own view is that the world may be deterministic, but I like to think I have free will. I believe that probability should be regarded only as a model of reality.